The present invention relates to a rubber composition suitable for use in treads of automobile tires, and more particularly to a rubber composition for use in treads of tires of low fuel cost having excellent wet skid characteristic, excellent rolling resistance characteristic and excellent skid characteristic on ice-snow road (hereinafter referred to as "ice skid characteristic").
In recent years, the reduction of running fuel cost of automobiles has been in strong demand increasingly, paritcularly it is of urgent demand to improve characteristics of tires, because it has an important influence on fuel saving.
The characteristics required for tires are principally wear resistance, wet skid characteristic, low heat generation, bending resistance, chipping resistance and groove cracking resistance, and they must be well balanced. Especially, from the viewpoint of resources and energy saving, it is important that energy loss and rolling resistance are low.
Among these characteristics, a high wet skid characteristic for steering stability and a low rolling resistance for fuel saving are particularly important, but the both characteristics have been recognized as contrary to each other from the conventional knowledge.
The present inventors, while continuing the fundamental study of the rolling and wet skid characteristics which have been hitherto considered as antinomic, made a study of the structure and polymerization process of polymers having an excellent processability practically required in tire manufacturing such as kneading processability, roll processability and extrusion processability, and they found, as described in U.S. Pat. No. 4,523,618, that a branched polymer having a particular structure has a high wet skid characteristic and an excellent rolling resistance together with an excellent processability.
Also, rubber compositions suitable for use in the treads of low fuel cost tires are similarly proposed by the present inventors, for instance, in Japanese Unexamined Patent Publication Nos. 4633/1984 and 4634/1984 and U.S. Pat. No. 4,482,678. These compositions contain as a main rubber component a copolymer of a vinyl aromatic compound and butadiene having a particular structure and a particular molecular weight distribution, and they have both a high wet skid characteristic and an excellent rolling resistance characteristic and furhter excellent processability and excellent practical moldability required in molding of tires.
Also, in recent years, since the public nuisance of dust particles due to the spike tires in a cold district is high-lighted and since exchanging summer tires with snow or spike tires is awkward, tires available throughout the year without exchanging is strongly demanded.
A tread rubber used in these tires requires an excellent ice skid characteristic in addition to a high wet skid characteristic and a low rolling resistance. In order to improve the skid characteristic on ice-snow road, it is known hitherto that a rubber hardness at a low temperature must be reduced, but it is difficult to satisfy these characteristics at the same time.
For instance, in a rubber composition mainly composed of an emulsion-polymerized SBR, a good wet skid characteristic may be obtained, but an energy loss is high and a rolling resistance characteristic is poor, besides an ice skid characteristic is also not sufficient.
Also, a rubber composition using rubber components having a low glass transition temperature (Tg) such as natural rubber, polyisoprene rubber, high-cis-1,4-polybutadiene rubber and the like, has superior rolling resistance characteristic and ice skid characteristic, but is poor in wet skid characteristic.
Further, though the compositions of the aforemetioned Japanese Unexamined Patent Publication Nos. 4633/1984 and 4634/1984, U.S. Pat. Nos. 4,523,618 and 4,482,678 have a high wet skid characteristic together with an excellent rolling resistance characteristic, the rubber hardness increases rapidly at a low temperature due to its relatively high Tg, thus the ice skid characteristic is far from the satisfied level.
Various rubber compositions are also proposed by other persons. For instance, in Japanese Unexamined Patent Publication Nos. 102912/1982 and 108142/1982, there are disclosed rubber compositions containing a rubber which is a styrene-butadiene copolymer prepared using an organolithium initiator and having two blocks with different bound styrene contents and vinyl contents in butadiene component. The copolymers disclosed in these publications are linear polymers. The compositions have well-balanced wet skid characteristic and rolling resistance characteristic, but the ice skid characteristic is not in a satisfactory level.
In Japanese Unexamined Patent Publication Nos. 165445/1982, 200439/1982 and 192739/1985, a rubber composition containing a branched styrene-butadiene block copolymer having two different blocks is disclosed. A relatively good ice skid characteristic is achieved, but a level required in practical use is still not satisfied.
Also, though it has been attempted to blend different kinds of polymers together so as to possibly harmonize the aforementioned characteristics which are conflicting with each other, it is the present situation that the demanded performance is not still satisfied and further improvements are strongly demanded in the industry, setting aside some improvements being made to a certain extent.
In view of the present situation, the present invention is intended to improve drawbacks of conventional rubber compositions for use in the tread of tires, in which a satisfactory level is not achieved in an ice skid characteristic.
Accordingly, it is an object of the present invention to provide a rubber composition suitable for use in the tread of low fuel cost tires, in which wet skid characteristic, rolling resistance characteristic and ice skid characteristic are totally improved and moreover which has an excellent processability.